Setting up Word Clock

in simple and advanced digital audio recording systems

by TweakHeadz Lab

The ART SyncGen is a simple to use and inexpensive way to improve
the sound of your digital recording gear and eliminate seemingly
random pops and clicks resulting from synchronization timing errors
between different pieces of equipment.

Designed to fit in a variety of digital audio situations,
the GENx192 provides an extremely low jitter master clock
reference in a variety of digital audio formats. Capable
of handling sampling frequencies up to 192kHz, this unit
will be a versatile companion in any digital audio application
including recording and broadcast scenarios. It offers an
extraordinary package in terms of jitter performance, connectivity
and flexibility, all at a reasonable price point.

As your digital audio rig
gets largeryou will inevitably
have to deal with word clock distribution. At first, synchronizing word clocks
of different digital audio devices is easy. In most cases, you can generate
and transmit the word clock signal along with the digital audio signals passing
from master to slave through s/pdif coax, Toslink and (sometimes)
ADAT optical cables and AES/EBU cables. But soon enough you
will run into a situation where syncing through audio cables will stop working.

That
is where you need to set up a way to distribute the word clock master to
all the slaves that need it. This is very simple stuff, so don't let it freak you.
But don't confuse word clock with MIDI clock synchronization, Midi time code, SMPTE,
or anything that regulates tempo. We'll talk about those in the next article.

Sample Words and the clock that regulates them

First, without getting too technical lets get into why
we need a word clock. Digital audio, as we know, is made up of
1s and 0s, or bits. These bits are grouped into sample words.
The word size for common digital audio is 16 bits, 24 bits and sometimes 32 bits
and even 64 bits. Indeed when we talk about 24 bit audio we mean data with
sample words of 24 registers for bits. When these sample words start and end
at the same exact time on several devices, we have word sync.
That is what we want.

The word clock sets the rate at which these
words are transmitted. This to takes us back to our old friend the sample
rate. We might use a clock rate of 32kH, 44.1 kHz, 48 kHz, 96 kHz, and
for those who have bought the hype 192 kHz.

When you use multiple digital audio devices they must use the
same size word and travel at the same speed. We are keeping this simple. (More
technical descrriptions) You might ask. What happens when you send a signal
at a clock of 48kHz into a device working at 44.1. If you are lucky,
it will just stop working. If not, you might find one of the devices explode
into an intensely loud cascade of white noise. That is what can happen during
an obviously big clock mismatch. (This is a good reason never to wear headphones
when messing with word clocks! Keep your system volume WAY down too.)

But what about when the clocks are very close, but not perfectly,
synchronized. Then we get what we can call "jitter".
Jitter may be subtle or extreme. At it's extreme, there will be distortion
that almost sounds like ring modulation in all the audio. Sometimes you will
hear the rhythmic pulsing of soft white noise bursts happening about a second apart.
(I usually hear that when I have 2 devices set to master.) As the rates get
closer this is more like loud popping and crackling noise in the audio which still
makes the audio unsuitable. As the rates get even closer, you might hear only
a few microscopic barely audible ticks once every interval, sometimes 5, 20, 50
seconds apart. Many of us may actually have systems ridden with jitter but
because the artifacts are so far apart we tend to ignore them like one would ignore
the occasional record pop on a vinyl record.

SuperClocks. How much is hype?

The professional has to be particularly careful about distributing
word clock because these ticks, even if far apart, are not acceptable.
These tics and pops not only exist in the playback of audio, but are burned into
the recorded data as well. While they can be surgically removed with audio
editors, it is much better to get it right the first time! There is also a bit of
audio voodoo here. Many professionals claim that even though there are no
audible artifacts of jitter, a less that perfectly stable clock can cause degradations
in clarity and on the width of the sound field. This leads them to buy special
word clock generators known to have excellent stability. I call them SuperClocks.
As you approach the high end digital audio converters, you will often see manufacturers
tout why their clocks are better than their competitors. Is this hype? Much
as with the claim that recording at 192kHz is "better", if there are differences
in quality we may not be able to perceive them. A professional might simply buy
a super word clock solution as insurance against jitter. Voodoo? Let your
ears decide.

Setting Up a word clock distribution system

The problem: You have one word clock out on the master device.
How do you get this one cable to go to all 3 slave machines?
First, there can be only one master clock source. All
other digital devices must be slaves. Second, word clock
is a one way system. it does not have to loop around back to the
Master's word clock in. Third, you can't assume that devices with a WC in
and WC out will pass the signal at the in to the out. The WC out may be reserved
only for signal internally transmitted.

Now lets apply a synchronized word clock to solve this problem.

There are two solutions here. 1) The more
expensive solution is to get a word clock generator with multiple BNC outputs.
Here the central device sends out the word clock to all four WC outputs. Each
device reads the clock, locks into sync, and audio data flows smoothly from one
device to the next.

Back of the
Lucid Genx192 Notice how the inputs can send signal to various formats
such as BNC, AES and s/pdif.

2) A second, less expensive method is to get several
BNC "Tee" adapters. Every time you need to add a digital device with word
clock, you get another cable and another Tee adapter and simply extend the system.
You want to observe the gender of your cables to make sure
you get the right ones. But I suggest the following rule. Because all
WC ins and outs are female, all WC cables you buy should be male to male.
That will mean that your Tee adapter should be male where you plug it into the unit
and have two female arms. (see pic). That way you connect your male
WC cables directly to each arm and on to the next device. Using the Tee basically
splits the signal into two, so one goes to the device and the other passes on to
the next.

To test your word clock connection just simply change the sample
rate on the master. All the slaves should simultaneously change their sample
rate. When you see that, you know you have wired the cables correctly.
Now make a recording from each digital device listen carefully over headphones on
playback for any evidence of ticks, pops, clicks, and more obvious forms of distortion.
You should hear perfectly clean audio. If you do you have successfully applied
a word clock distribution system. Congratulations.

Questions and Answers

Q) What is s/pdif, toslink, ADAT and AES/EBU

A) All of those a methods of transmitting digital audio data
from device to device. Note that digital audio data is your audio signal
after it has been converted to ones and zeros.

Q) Can you make your own Word Clock cables?

A) Yes. The cable is good old 75ohm cable just like your
TV cable that has "F" connectors. Just go to an electronic store (or use my
links to Amazon) and pick up some Female "F" to male BNC connectors that can be
screwed on. Many video RCA cables are also 75ohm. With these just buy
some Female RCA to Male BNC adapters. Coaxial s/pdif cable 75 ohm RCA cable.
That can be used as well. BNC cables are used in professional broadcast systems
and in home video surveillance systems. Places that serve these customers
are more likely to have the adapters than your local music store.

Q) Is there any rule of thumb for deciding which device
should be the master?

A) Yes. The device that does the most critical analog to
digital conversion should be the master, when possible. The theory is that
the "native" clock on the device doing the conversion will be better. However,
a stable clock is better than an unstable clock, so in some situations the native
clock rule does not hold.